Block copolymers have been developed rapidly within the recent past, the starting monomers usually being monoalkenyl arenes such as styrene or alpha methyl styrene block polymerized with conjugated dienes such as butadiene and isoprene. A typical block copolymer of this type is represented by the structure polystyrene-polybutadiene-polystyrene. When the monoalkenyl arene blocks comprise less than about 55% by weight of the block copolymer, the product is essentially elastomeric. Moreover, due to their peculiar set of physical properties they can be referred to more properly as thermoplastic elastomers. By this is meant polymers which in the melt state are processable in ordinary thermoplastic processing equipment but in the solid state behave like chemically vulcanized rubber without chemical vulcanization having been effected. Polymers of this type are highly useful in that the vulcanization step is eliminated and, contrary to scrap from vulcanized rubbers, the scrap from the processing of thermoplastic elastomers can be recycled for further use.
When the proportion of monoalkenyl arenes is increased beyond about 55 wt % of the polymer up to about 90% by weight of the polymer, the resulting product resembles a high impact thermoplastic material. Those block polymers which comprise in part conjugated diene polymer blocks have one substantial shortcoming, namely, their susceptibility to oxidation or ozonolysis. Substantial improvements both in stability and compatibility with alpha-olefin polymers have been made by hydrogenation of such block polymers. The hydrogenation may be non-selective, selective or complete. Certain technical advantages have been found for selective hydrogenation wherein at least about 80% of the aliphatic double bonds are reduced and no more than about 25% of the aromatic double bonds are reduced by hydrogenation.
While these selectively hydrogenated block copolymers have improved stability over their unsaturated precursors, they have certain shortcomings which it would be desirable to eliminate or minimize. Chief among these is lessened processability. It is possible, of course, to improve processability by diluting the polymer with extending oils and the like. This normally results in a reduction in other physical properties, particularly tensile strength and properties associated therewith.
It is an object of the present invention to improve block copolymer compositions. It is a further object of the invention to provide such compositions which have a substantially improved processability and high tensile strength as well as other associated physical properties. It is a further object of this invention to introduce polar groups into the composition so as to improve properties such as adhesion of the composition to polar substrates. Other objects will become apparent during the following detailed description of the invention.